I have to validate the actual ET obtained from model output by using the actual ET value derived from the pan evaporation value. Is there a method to calculate the daily AET from pan evaporation?
Potential evapotranspiration can be obtained using a locally reliable plant coefficient (Kc) by the following equation :
ETp = Kc * ETpan
but field observational data are needed to estimate Actual evapotranspiration (ETa)
the daily evapotransipration is : ET= Kp * ETpan
Kb is between 0.7 and 0.8
Thank you Amirmohamad Abasi , Amlal Fouad for your answers.
I would like to know the relationship between pan evaporation and daily actual evapotranspiration.
Dear Thejal
I send you a piece of FAO56 manual where is the method to calculate ET from pan evaporation.
Please note that the relationship between reference crop evapotranspiration (ETo) and pan evaporation (Ep) is well known and you can estimate ETo using a proper pan-crop coefficient.
ETo = Kpc x Ep.
Where, ETo- reference evapotranspiration (mm/day); Kpc- pan-crop coefficient, and Ep- pan evaporation (mm/day).
If you want to calculate actual crop evapotranspiration (ETc), you have to multiply ETo with another coefficient, Kc (crop coefficient).
ETc = Kpc X Ep X Kc
Analysis of data on ETc indicates a high degree of correlation between pan evaporation values and crop evapotranspiration. This indicates the possibility of using pan evaporation values directly to estimate crop evapotranspiration (ETc) employing a coefficient called crop factor (Cf). The crop factor accounts for both Kpc and Kc. Therefore, if you can estimate the values of “crop factors” locally, you are freed of the burden of looking for pan coefficients and crop coefficients. The crop factor (Cf) varies with crop type and crop growth stages. This method is also known as Hargreaves Class A Pan evaporation method (Hargreaves, 1968). The relationship between crop evapotranspiration (ETc) and pan evaporation (Ep) is usually expressed as:
ETc= Ep X Cf
Where, ETc - crop evapotranspiration (mm/day); Cf - crop factor for the period; and Ep- pan evaporation (mm/day).
Estimation of daily ETc relies on daily readings of pan evaporation from a suitably placed evaporation pan together with an appropriate set of crop factors. Approximate value of crop factors were given by Hargreaves(1968). However, for accuracy, you have to find local Cf values. For using this method, the user has to enter daily readings of pan evaporation to calculate the crop water use for the day.
Reference: Hargreaves, G.H. 1968. Consumptive use derived from evaporation pan data. J. Irrig. Drain. Div. (American Society of Civil Engineers) 94(1): 97-104.
I invite all of to have a review of my latest book “Irrigation and Water Management”, where issues of this kind are discussed in a contemporary style. Please visit
https://www.researchgate.net/publication/354616406_Irrigation_and_Water_Managementor or
www.anebooks.com
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